1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display including a pixel array (so-called of “staggered structure”) in which pixel electrodes belonging to the same scanning line are not assigned to the same straight line but staggered up and down in a zigzag pattern, thereby operating in a quasi dot inversion driving method.
2. Background Art
A conventional active-matrix liquid crystal panel includes two transparent substrates sandwiching a liquid crystal layer in between. One of the substrates has a surface formed with a plurality of data lines (may also be called “data signal line” or “column electrodes”), and a plurality of scanning signal lines (may also be called “row electrodes”) intersecting the data lines. For each of the intersections, a pixel electrode is formed, to form a matrix of the pixel electrodes. Each pixel electrode is connected to the data line that passes the intersection relevant to the electrode, via a TFT (Thin Film Transistor) serving as a switching element. The TFT has its gate terminal connected to the scanning signal line that passes this intersection. The other transparent substrate is formed with a counter electrode that is common to the pixel electrodes. A liquid crystal display including a liquid crystal panel having a structure such as the above further includes a drive circuit for displaying images in the liquid crystal panel. The drive circuit includes a row electrode driving circuit (may also called “scanning line driving circuit” or “scan driver”) that supplies the scanning signal lines with scanning signals for alternate and sequential selection of a line from the scanning signal lines, and a column electrode driving circuit (may also called “signal line driving circuit” or “data driver”) that supplies the data lines with data signals for writing data onto each pixel-forming portion in the liquid crystal panel. With such a configuration, a voltage is applied between each pixel electrode and the counter electrode, at a value representing the value of the corresponding pixel. The applied voltage changes transmissivity of the liquid crystal layer, forming a desired image in the liquid crystal panel. In this operation, in order to protect liquid crystals in the liquid crystal layer from deterioration, the liquid crystal panel is driven by an alternating current. Specifically, the polarity of the voltage applied between each pixel electrode and the counter electrode is inverted for every frame for example, by the data signals outputted from the column electrode driving circuit.
Generally in active-type liquid crystal panels, the switching element such as the TFT provided for each of the pixels does not have perfect characteristics, and therefore, even if the column electrode driving circuit outputs symmetric data signals having a symmetric waveform in the positive and the negative phases (with respect to the baseline provided by an electric potential of the counter electrode), the liquid crystal layer does not respond in perfect symmetry in terms of transmissivity change to the positive and the negative data voltages. This causes flicker in the liquid crystal panel display if the display uses a driving method in which the positive and negative poles of the voltage applied to the liquid crystal are alternated for each frame (single-frame inversion driving method).
In an attempt to reduce such a flicker, there is disclosed a method in which the voltage polarity is inversed for each horizontal scanning line and for each frame (hereinafter called “1-H inversion driving method”). There is another method known in which the polarity of voltage applied to the liquid crystal layer or the pixels is inverted for each scanning signal line, each data line, and each frame (hereinafter called “dot inversion driving method). Now, in comparison, the dot inversion driving method is obviously superior to the 1-H inversion driving method in flicker reduction capability. Further, the 1-H inversion driving method has another disadvantage that when the user is following a moving image on the screen, he often perceives horizontal line marks.
As described, in view of display quality, the dot inversion driving method is advantageous over the 1-H inversion driving method. However, the 1-H inversion driving method offers an advantage that the column electrode driving circuit can be provided by an IC (integrated circuit) having a low withstand voltage since the voltage of the counter electrode (common electrode) is alternated for each horizontal scanning period. On the contrary, in the dot inversion driving method, when a positive voltage is applied to a pixel electrode in a given horizontal scanning line (a given line on the pixel matrix), a negative voltage is applied simultaneously to another pixel in the same line. For this reason, the IC for the column electrode driving circuit must have a high withstand voltage.
Under these circumstances, efforts are being made to achieve a seeming dot inversion driving method at a low withstand voltage, using the IC that is normally employed for the column electrode driving circuit operating in the 1-H inversion driving method. Thus, there is proposed a liquid crystal panel having a staggered structure as shown in FIG. 19A and FIG. 19B. Specifically, according to this liquid crystal panel which includes a matrix of pixel electrodes, the pixel electrodes sharing the same scanning signal line via TFTs are not assigned to a single line on the pixel matrix, but assigned in a dispersed manner to two mutually adjacent upper and lower lines, in an up-and-down zigzag pattern.
For example, the Japanese Patent Laid-Open No. 4-309926 discloses a liquid crystal display comprising a matrix array of display pixels each including a liquid crystal cell and a switching element. The display pixels in each of the columns and rows are connected by a grid of signal lines and scanning lines intersecting generally perpendicularly to each other. With this arrangement, the pixels driven by the above-described same scanning line are staggered up and down at least for every pixel of the signal line. The gazette further discloses a function of this liquid crystal display as follows: Pixels driven by the driving elements are staggered by one scanning line for every pixel of the signal line, and therefore, a normal flicker-less operation, in which the voltage polarity is altered for every scanning line, gives a perception as if inversion is made for every pixel, resulting in reduced perception of the vertical and the horizontal line marks.
However, such a seeming dot inversion driving as described above (hereinafter called quasi dot inversion driving), still leaves problems to be solved on the quality of display. Specifically, when a Windows (trademark) operating system displays a checker pattern as shown in FIG. 24A called “checker back” when closing the system for example, the pattern is accompanied by a stripe pattern extending vertically (hereinafter called “vertical shadow”) in the above conventional liquid crystal display which utilizes the quasi dot inversion driving method. It should be noted that this vertical shadow also appears on the screen which is not driven in the quasi dot inversion driving method but in a genuine dot inversion driving method (hereinafter called “true dot inversion driving method”). For clarification of the problem, description will be made here below on why the vertical shadow appears in the quasi dot inversion driving method and the true dot inversion driving method.
As shown in FIG. 19C, each pixel-forming portion in the liquid crystal panel is sandwiched by two data lines Lss and Lsn, and includes: a TFT having its gate terminal connected to a scanning signal line Lg; a pixel electrode Ep connected to the data line Lss via the TFT; and a counter electrode Ec common to all of the pixel-forming portions. In this construction, one of the two data lines serves as a data line for writing data onto the pixel-forming portion (more specifically into a pixel capacity Cp formed by the pixel electrode Ep and the counter electrode Ec). This is the data line Lss (and hereinafter called “feeding data line”). Now, between the feeding data line Lss and the pixel electrode Ep of the pixel-forming portion, there is a parasitic capacity (hereinafter called “Csd (self)”). Similarly, between the other data line Lsn (hereinafter called “adjacent data line”) and the pixel electrode Ep of the pixel-forming portion, there is another parasitic capacity (hereinafter called “Csd (other)”). For this reason, in each pixel after a data is written onto the pixel-forming portion of the pixel (i.e. when TFT is off), the value in this pixel is influenced by signal change in the data line Lss via Csd (self), and by signal change in the data line Lsn via Csd (other). Here below, description will be made on the basis that the signal changes in these feeding data line Lss and adjacent data line Lsn cause the vertical shadow. Note also that because Csd (self) is generally equal to Csd (other), the description will be made on the following premise: Csd (self)=Csd (other).
<Quasi Dot Inversion Driving Method>
First, a study will be made to a case in which the “checker back” is displayed in an active-matrix liquid crystal panel having a staggered structure such as shown in FIG. 19A-19C, and driven in the quasi dot inversion driving method. FIG. 19A schematically shows a construction of the liquid crystal panel. FIG. 19B shows an equivalent circuit for a portion 810 which includes 2×2 pixels on the liquid crystal panel shown in FIG. 19A. FIG. 19C shows an equivalent circuit for a portion including a single pixel in the liquid crystal panel, with the parasitic capacities illustrated in the drawing.
In this case, in one frame (period) F1, the “checker back” is displayed under a polarity pattern as shown in FIG. 20A, and in the next frame F2, the “checker back” is displayed under a polarity pattern as shown in FIG. 20B. For the sake of description, the example will use only five effective horizontal scanning lines and six data lines. (Note, however, that in the staggered structure, the number of scanning signal lines is six, or one more than the number of horizontal scanning lines effective for the display.) Note further, that in FIGS. 20A and 20B, crosshatched pixel-forming portions indicate that the display is made in black whereas pixel-forming portions without the crosshatch indicate that the display is made in white. The display is made by using a unit of display provided by mutually adjacent three pixels representing the colors of R (red), G (green) and B (blue). A block of white and a block of black are alternated with each other in the horizontal and the vertical directions. Reference symbols R1, G1, B1 and R2, G2, B2 each represents data signal fed to a corresponding one of the six data lines, and at the same time represents the corresponding column of the pixel-forming portion (hereinafter may also be called “pixel column” for convenience). (The premises given above for the study of the appearance mechanism of the vertical shadow will also apply in later discussions.)
In this case, the data signals G1, B1, R2 change as shown in FIGS. 20C, 20D and 20E respectively, with the baseline provided by an electric potential at the counter electrode Ec. In these FIGS. 20C-20E, “+V1” and “−V1” represent a positive voltage and a negative voltage respectively, to be applied to apart of the liquid crystal layer that forms the pixel-forming portion (hereinafter called “pixel liquid-crystal”) for displaying the color of white. Likewise, “+V2” and “−V2” represent a positive voltage and a negative voltage respectively, to be applied to the pixel liquid-crystal for displaying the color of black. (This applies to all of the drawings hereinafter.) As already mentioned, “F1” and “F2” represent two sequential frames. “S1”-“S6” represent periods for which respective scanning signals SS1-SS6 shown in FIGS. 20A and 20B become active, i.e. the horizontal scanning periods in a frame.
Now, attention will be paid to a pixel-forming portion (hereinafter may also called “pixel” for simplicity) in the first row of Column G1. This pixel has a feeding data line Lss carrying a signal G1, and an adjacent data line Lsn carrying a signal B1 (See FIG. 19C and FIG. 20A). To this pixel, a data (−V2) is written in the horizontal scanning period S1 in frame F1. Now, the value of this pixel (the written value) is influenced by signal changes in the two data lines Lss, Lsn, to the extent (direction and magnitude) determined by the amounts of signal changes in the two data lines with respect to baselines provided by signal values of the feeding data line Lss and the adjacent data line Lsn respectively at the time the writing was made. Thus, here below, the amounts of signal change in the two data lines will be obtained with reference to FIGS. 20C-20E, with the baselines given by signal G1 (−V2) for the feeding data line and by signal B2 (−V1) for the adjacent data line.
Writing is made to the pixel of attention in the horizontal scanning period S1 in frame F1. During this period, obviously, the amount of signal change is zero in both of the feeding data line (signal G1) and the adjacent data line (signal B1). On the other hand, with a shift of the signal scanning period from S1 to S2, signal G1 changes from −V2 to +V1 whereas signal B1 changes from −V1 to +V2. Thus, the amount of signal change is +(V1+V2) in both of the feeding data line and the adjacent data line. Further, in the next horizontal scanning period S3, signal G1=−V2 and signal B1=−V1, i.e. the values come back to the signal values at the time the writing was made to this specific pixel. Therefore, the amount of signal change is zero in both of the feeding data line and the adjacent data line. Further, in the next horizontal scanning period S4, signal G1=+V1 and signal B1=+V2. Therefore, the amount of signal change is +(V1+V2) in both of the feeding data line and the adjacent data line, with respect to the baseline signal values (G1=−V2, B1=−V1) at the time the writing was made to this pixel. Likewise, the amount of signal change becomes zero in both of the feeding data line and the adjacent data line in the horizontal scanning period S5, and then becomes +(V1+V2) in both of the data lines in the horizontal scanning period S6.
After the frame is switched, i.e. in frame F2, a new pixel data is written in the horizontal scanning period S1 of frame F2. For the period of frame F2, attention will be paid to the pixel in the fifth row of Column G1 (the last pixel into which the data is written in frame F2), and a study will be made on how the value of this new pixel is influenced by signal changes in the feeding data line and in the adjacent data line. In this case, the amounts of signal change in the two data lines will be obtained in the same way as described above, with the baselines provided by a value of signal G1 (−V2) in the feeding data line and a value of signal B1 (−V1) in the adjacent data line at the time when the writing was made to the pixel in Column G1, Row 5 (in the horizontal scanning period S5 of frame F1). Specifically, with reference to FIGS. 20C and 20D, in the horizontal scanning period S1 of frame F2, the amount of signal change in the feeding data line (signal G1) is +2V2 whereas the amount of signal change in the adjacent data line (signal B1) is +2V1. In the horizontal scanning period S2, the amount of signal change in the feeding data line is +(V2−V1) whereas the amount of signal change in the adjacent data line is −(V2−V1). In the horizontal scanning period S3, the amount of signal change in the feeding data line is +V2 whereas the amount of signal change in the adjacent data line is +V1. In the horizontal scanning period S4, the amount of signal change in the feeding data line is +(V2−V1) whereas the amount of signal change in the adjacent data line is −(V2−V1). In the horizontal scanning period S5, the amount of signal change in the feeding data line is +V2 whereas the amount of signal change in the adjacent data line is +V1. In the horizontal scanning period S6, the amount of signal change in the feeding data line is +(V2−V1) whereas the amount of signal change in the adjacent data line is −(V2−V1).
As described above, with attention paid to pixels in Column G1, the amounts of signal change in the feeding data line and in the adjacent data line are as shown in FIG. 21A (not all data are shown), with respect to the baselines given by signal values in the respective data lines at the time the writing was made into these pixels. (Note that the pixel of attention in frame F1 is different from the one in frame F2.)
Next, attention will be paid to the borderline portion between the display units in black and the display units in white in the “checker back”, i.e. to pixels in Column B1 (rows 1 and 5). For these pixels, the signal in their feeding data line Lss is B1 whereas the signal in the adjacent data line Lsn is R2. In this case, with reference to FIGS. 20D and 20E, the amounts of signal change in the feeding data line and in the adjacent data line are as shown in FIG. 21B, with respect to the baselines provided by signal values in the feeding data line and in the adjacent data line at the time when the writing was made to these pixels.
Now, attention will be turned to the pixels in Column G1. As shown in FIG. 21A, in frame F1 (before the frame is switched), signal changes are positive in both of the feeding data line and in the adjacent data line. This influences the pixel of attention (Column G1, Row 1) in such away that its value (−V2) is increased. On the other hand, when attention is turned to the pixel in Column B1, as shown in FIG. 21B, signal change is negative in both of the feeding data line and the adjacent data line in frame F1 (before the frame is switched). This influences the pixel of attention (Column B1, Row 1) in such a way that its value (+V2) is decreased. As described, in Column G1 and Column B1, the pixels of attention have the same absolute value but with different signs (−V2 vs. +V2) and accordingly, their amounts of signal change also have different signs (+(V1+V2) vs. −(V1+V2)). However, the changes are equal to each other in the absolute value, and hence will cause the same influence on a displayed image.
On the contrary, in frame F2 (after the frame is switched), as will be clearly understood by comparing the amounts of signal changes shown in FIG. 21A and in FIG. 21B, the pixel of attention in Column G1 (Row 5) and that in Column B1 (Row 5) are influenced differently by the signal changes in the feeding data line and in the adjacent data line. Specifically, after the frame is switched, the pixels of attention in Column G1 and in Column B1 are influenced in the same direction, or in such a way that their absolute values (−V2 vs. +V2) are generally decreased. Taking the following fact into consideration that V2 is sufficiently greater than V1, the pixels in Column B1 will be influenced to a greater extent than the pixels in Column G1. The pixels in Column R1 will be influenced practically as much as those in Column G1. Therefore, areas such as Column B1 that is influenced strongly by signal changes in the feeding data line and in the adjacent data line, i.e. the borderline area in the “checker back”, will show the vertical shadow.
<True Dot Inversion Driving Method>
Next, a study will be made to displaying the “checker back” by way of the true dot inversion driving method in an active-matrix liquid crystal panel that has a common, non-staggered, structure. In this case, in one frame F1, the “checker back” is displayed under a polarity pattern as shown in FIG. 22A whereas in the next frame F2, the “checker back” is displayed under a polarity pattern as shown in FIG. 22B. Note that in this example, the liquid crystal panel does not have a staggered structure, and therefore, the effective number of the horizontal scanning lines is equal to the number of the scanning signal lines, which is five.
In this case, data signals G1, B1, R2 change as shown in FIGS. 22C-22E, with respect to the baseline provided by the counter electrode Ec. In these FIGS. 22C-22E, S1-S5 represent periods for which respective scanning signals SS1-SS5 shown in FIGS. 22A and 22B become active, i.e. the horizontal scanning periods in a frame. Here below, with reference to FIGS. 22C-22E, consideration will be made on how pixels of attention will be influenced by signal changes in the feeding data line and the adjacent data line.
First, as was in the above study on the quasi dot inversion driving method, consider influences on the pixels in Column G1 by signal changes in the feeding data line and the adjacent data line. For this purpose, attention is first made to a pixel in the first row of Column G1, and amounts of signal change in the two data lines will be obtained, with the baselines provided by a value (−V2) of signal G1 in the feeding data line and by a value (+V2) of signal B1 in the adjacent data line at the time when the writing was made into this pixel (in the horizontal scanning period S1 in frame F1). Next, attention will be paid to a pixel in the fifth row of Column G1, and amounts of signal change in the two data lines in frame F2 will be obtained, with the baselines given by a value (−V2) of signal G1 in the feeding data line and by a value (+V2) of signal B1 in the adjacent data line at the time when the writing was made into this pixel (in the horizontal scanning period S5 in frame F1). FIG. 23A shows these amounts of signal change in frames F1 and F2 obtained as described above. (Not all data are shown.)
Next, as was in the above study on the quasi dot inversion driving method, attention will be paid to pixels in Column B1 located on the borderline portion between the display units in black and the display units in white in the “checker back”, and consideration will be made to influences on pixel values by signal changes in the feeding data line and the adjacent data line. For this purpose, attention is first made to a pixel in the first row of Column B1, and amounts of signal change in the two data lines will be obtained, with the baselines provided by a value (+V2) of signal B1 in the feeding data line and by a value (−V1) of signal R2 in the adjacent data line at the time when the writing was made into this pixel (in the horizontal scanning period S1 in frame F1). Next, attention will be paid to a pixel in the fifth row of Column B1, and amounts of signal change in the two data lines in frame F2 will be obtained, with the baselines given by a value (+V2) of signal B1 in the feeding data line and by a value (−V1) of signal R2 in the adjacent data line at the time when the writing was made into this pixel (in the horizontal scanning period S5 in frame F1). FIG. 23B shows these amounts of signal change in frames F1 and F2 obtained as described above. (Not all data are shown.)
Now, attention will be turned to the pixels in Column G1. As shown in FIG. 23A, in both of frames F1 and F2 (before and after the frame is switched), signal G1 in the feeding data line and signal B1 in the adjacent data line change in a “complementary” pattern. Specifically, with respect to the baseline provided by a signal value in relevant data lines at the time the writing was made into the pixel of attention, the signal values (voltage values) of the two data lines are in a relationship that when one increases the other decreases, and the amount of increase and the amount of decrease are the same in the absolute value. Therefore, influences from the two data lines via the respective parasitic capacities Csd (self) and Csd (other) canceled each other, and as a result, signal changes in the two data lines do not influence the value of the pixel of attention in Column G1.
Now, attention is turned to the pixels in Column B1. As shown in FIG. 23B, in frame F1 (before the frame is switched), signal B1 in the feeding data line and signal R2 in the adjacent data line change in a complementary pattern. However, in frame F2 (after the frame is switched), signals B1 and R2 in the two data lines do not change in a complementary pattern. Therefore, signal changes in the two data lines influences the value of the pixel in Column B1, via the parasitic capacities Csd (self) and Csd (other).
As described above, values of the pixels in Column G1 hold at intended values (as well as values of the pixels in Column R1), whereas values of the pixels in Column B1 on the border region in the “checker back” are altered from intended values, resulting in appearance of the vertical shadow in the liquid crystal panel screen.
<Summary of the Problems>
As described above, when a dot inversion driving method is utilized, even if the dot inversion driving method is a true dot inversion driving method, the vertical shadow appears when the “checker back” is displayed. In other words, whether the method is the quasi dot inversion driving method or the true dot inversion driving method, the “checker back” is a “killer pattern” that causes a problematic display pattern such as the vertical shadow. Although the driving method ideally should not have such a killer pattern, it is difficult at a practical level to realize a liquid crystal panel or a liquid crystal display based on such an ideal driving method. Also, as has been mentioned earlier, in view of practicability of the driving method, the quasi dot inversion driving method is advantageous over the true dot inversion driving method in that the withstand voltage required of the IC for the driving circuit can be lower in the quasi dot inversion driving method.